1. Field of the Invention
The present invention relates generally to shotgun ammunition, and particularly to a system and method of sealing the launching end of a shotgun cartridge.
2. Description of the Related Art
The separate components comprising a conventional shotgun cartridge are shown FIG. 1A. Specifically, FIG. 1A shows the metal base 10 attached to the plastic envelope 20 which, when constructed, holds the propellant charge 30 tamped in the bottom of the cartridge next to metal base 10. In the center of metal base 10 is the primer which ignites the propellant charge 30 when struck by the hammer of the shotgun. The propellant charge 30 has a cushioning wad 40 also holding it in and separating the propellant charge 30 from the payload of this cartridge, which, in this case, is to be a less lethal projectile 50 comprised of a tubular sock-like body of fabric construction into which ballast material, such as lead shot, is filled, before the open end of the tubular sock-like body is tied off, as shown in FIG. 1A. The less lethal projectile 50 is held in the plastic envelope 20 of the cartridge by closure wad 60.
The launching end of the cartridge's plastic envelope 20 is typically sealed by either roll crimping or star crimping. FIG. 1B shows a sealed cartridge where roll crimping was used, i.e., the end walls of plastic envelope 20 have been rolled down in order to hold in closure wad 60 which can be seen at the launching end of the cartridge. FIG. 1C shows a sealed cartridge where star crimping was used, i.e., the end walls of plastic envelope 20 have been crimped, or bent, inwards in order to form a star-shaped seal at the launching end of the shotgun cartridge. When using a star crimp, it is possible to not use a closure wad 60, depending on the contents of the cartridge.
Plastic envelope 20 is typically made of high density polyethylene (HDP), although any appropriate material may be used, as is known to one skilled in the art. Propellant charge 30 is typically conventional black powder, although, once again, any appropriate propellant mixture may be used, as is known to one skilled in the art. In general, when any particular material is described in this application, it should be understood as an example, i.e., any material capable of performing the same function in the same manner is intended for use with the present invention. Similarly, when any particular method step is described in this application, it should be understood as exemplary, i.e., any manner of achieving the same function and/or result is intended to be covered by the present invention. Furthermore, it should be understood that a shotgun cartridge having a less lethal projectile as a payload is used as an example herein, and that the present invention may be applied to any type of cartridge, e.g., for a rifled or smooth-bore barrel, with or without a payload, which requires sealing of its launching end.
In the conventional shotgun cartridge, cushioning wad 40 is typically constructed of plastic, while closure wad 60 is typically constructed of plastic or fiber (e.g., paper). It should be noted that the prior art contains a wide variety of “wad” constructions, from simple discs of paper to plastic cylindrical containers which also hold at least part of a payload to more complex structures which completely surround and contain the payload (see, e.g., the wad in FIG. 3 of U.S. Pat. No. 4,164,903 to Bouza et al., the wad 5 in FIGS. 1 and 2 of U.S. Pat. No. 4,506,605 to Maki, or the wad 22 in FIG. 1 of U.S. Pat. No. 6,260,484 to Billings, all of which are incorporated by reference in their entirety). Although only the roll and star crimps are shown in FIGS. 1B and 1C, respectively, any sort of crimping can be used to seal the launching end of the shotgun cartridge. Moreover, it is possible for a shotgun cartridge to have neither a closure wad per se nor a crimped launching end of the cartridge (see, e.g., the '903 patent cited above).
Further details concerning the exemplary prior art shotgun cartridge construction shown in FIGS. 1A-1B-1C can be found in U.S. Pat. No. 6,202,562 to Brunn et al. (“Method of Preparing a Low Lethality Projectile For Flight”), which is hereby incorporated by reference in its entirety. Some of the purposes, usage, and functioning of such a less lethal projectile may also be found therein. However, this description of an exemplary prior art less lethal projectile cartridge is not intended to limit the possible embodiments of the present invention described herein.
There is at least one problem with the typical crimping of the conventional shotgun cartridge: the seal formed by the crimping may allow material from the external environment into the interior cartridge. For example, if exposed to rain, water may leak directly into the cartridge through the small hole (shown in FIG. 1C) which is typically formed at the center of the star crimp. If there is no closure wad 60, there will be nothing to stop the water. Even if there is a closure wad 60, it may be made of paper or cardboard, which may become sodden, thereby allowing moisture into the interior of the cartridge (and perhaps also causing the closure wad to disintegrate). Even if the closure wad 60 is made of a water-impermeable material, such as HDP, the friction seal made by the closure wad 60 and the star crimped or roll crimped material of the plastic envelope 20 will not be tight enough to stop fluids such as water from seeping through at least some of the areas of surface contact between the closure wad 60 and the plastic envelope 20.
Furthermore, the typical material used for the plastic envelope 20 of the cartridge, HDP, is particularly difficult to adhere to, and, accordingly, glues, pastes, and other binding agents are not useful if one attempts to glue the closure wad 60 and the plastic envelope 20 together. This difficulty in adhesion also means that a tight friction seal between the closure wad 60 and the plastic envelope 20 requires a great deal of force to create, and, once created, it is likely that a fluid would still be able to seep in through at least one permeation in the seal between the closure wad 60 and the plastic envelope 20.
Although the HDP of the plastic envelope 20 can be heat soldered (this is the way the plastic envelope is attached to the metal base 10 of the cartridge before the primer or propellant charges are inserted), heat soldering the launching end of the plastic envelope 20 once the cartridge contents have been inserted presents an obvious hazard.
The prior art has largely ignored the problems with forming a tight seal at the launching end of the cartridge. In fact, when the prior art refers at all to forming a tight seal, the seal being referred to is between the plastic envelope 20 and the cushioning or obturating wad 40 between the propellant charge 30 and the payload 50. For example, U.S. Pat. No. 4,506,605 to Maki (the '605 patent) uses an annular projecting rim on a solid plastic obturating wad to form a seal between the wad and the inner surface of the cartridge. Specifically, as shown in FIGS. 2A and 2B (which are reproductions of FIGS. 1 and 2 in the '605 patent), the rigid wad 5 as shown alone in FIG. 2B comprises a cylindrical container 14 for shot (only shown lining the circumference of wad 5 in FIG. 2A) with V-shaped notches 15 as well as a base with a concave portion 10 which faces the propellant charge 4. The annular projection 5a of the base has a greater diameter than the remaining portions of wad 5. As shown in FIG. 2A, this annular projection 5a of the wad forms a seal with the inner surface of cartridge case in order “to prevent leakage of gases when the wad is located within the case 1 and which comes into contact with the inner surface 20 of the barrel 21 (FIG. 4) during the passage of the wad through the barrel” (col. 3, lines 3-8, the '605 patent).
In other words, the prior art wads that are directed to forming a tighter seal are directed to forming a tighter seal inside the cartridge to prevent the “leakage of gases” when the propellant charge ignites. These pieces of prior art barely mention the seal at the launching end of the cartridge: the '605 patent merely states that “. . . a quantity of pellets 6 is packed into the container 14 and are blocked with the paper disc cover 8 fitted to the opening end of the container 14 which is locked by crimp 7, or stopped by a so called starcrimp formed by folding the top end of the case inwardly” (col. 4, lines 46-51, the '605 patent). Thus, in the prior art, different wads forming different sorts of seals to keep in or otherwise manipulate the igniting propellant charge or flying payload are discussed, but the seal at the outer launching end of the cartridge is not described or discussed in any detail (see, e.g., U.S. Pat. No. 3,974,770 to Clark, Jr.; U.S. Pat. No. 4,574,701 to Fiocchi; U.S. Pat. No. 5,408,931 to Tallman; and U.S. Pat. No. 5,361,701 to Stevens, all of which are incorporated by reference in their entirety).
Although some of these prior art seals, such as the one formed by rigid wad 5 and the inner surface of the shotgun cartridge shown in FIG. 2A, may possibly protect the propellant charge from moisture which has entered the cartridge through the launching end of the cartridge, they provide no protection for the payload which is located above the obturating wad. In the case of shotgun launcher cartridges, which have a propellant charge but no payload (because such cartridges are used to “launch” a payload, such as a grenade, attached at the muzzle end of the shotgun barrel), the moisture coming in through the launching end of the cartridge could completely neutralize the propellant charge. Furthermore, the remnants of the closure wad of a shotgun launcher cartridge can be expelled when the launcher charge is ignited, possibly striking or landing on a nearby object or person.
Therefore, there is a need for a method and system for sealing the launching end of a cartridge, such as a shotgun shell, which effectively seals the contents therein from the external environment and, in the case of a shotgun launcher cartridge, prevents the ejection of the detritus left from the closure wad.